Positioning devices of this kind are e.g. used in machine tools for continuously positioning the tool in relation to the machined workpiece (see e.g. DE 199 48 490 A1). The use of a linear motor drive offers the advantage that its operation is contactless and that high speeds and accelerations can be produced, thereby allowing rapid movements of the carriage. One disadvantage, however, is that the attractive force created between the primary part and the secondary part of the linear motor may be so high that the carriage is subject to deformations and can no longer be positioned with sufficient accuracy. In circular grinding, for example, a highly precise positioning of the tool with accuracies in the submicrometer range is required.
The patent application DE 196 43 518 A1 describes a positioning device with a linear drive comprising a horizontal and a vertical aligned active unit. The passive units of the linear drive are attached to a body which has a rectangular cross-section and which is displaceable by means of an air bearing. This kind of bearing, however, is relatively complicated and costly, since a plurality of nozzle has to be arranged over the whole surface of the active units. Furthermore, the carrying force of the air bearing is limited.